1. Field of the Invention
This invention relates to varying the video contrast of a video image and particularly to a system and method for automatically increasing the dynamic range of contrast of an input sequence of pixels or data points representative of a video image.
2. Prior Art Statement
In the art of video display systems, limitations occur in the quality of the video signal display. One source of these limitations may occur in the display circuits themselves, which may be subjected to video signals of a dynamic range beyond that of the video equipment: weak video signals may fall below a limited threshold as to be denied processing for display, while other signal portions may saturate the system. Another source of display signal limitation may be due to the poor quality of the signal itself, whereby increased gray-tone discrimination is desired in order to obtain adequate image detail intelligence from the video signal display.
Various systems and means have been proposed in the prior art for the enhancement of video signals. An early effort in this regard is set forth in U.S. Pat. No. 3,257,506 to Siepmann for controlling the contrast in television video signals, in which contrast control is applied to a selected portion or area of the video display image by means of a contrast control signal applied to either a sensitivity control circuit of the TV camera system or an electrode of the display tubes.
Such analog system, in being directed to only a portion of an entire image field and being developed from a separate, limited field of view scanner, does not provide enhancement over an entire field of view. Also, because the control voltage is derived as a function of the average video signal level over the selected partial field of view, such scheme does not lend itself to controlling image contrast over an entire field of view, especially one displaying a wide dynamic range of video signals.
Two, more-recent, references in the art of video signal enhancement are U.S. Pat. No. 3,983,320 to Ketchem et al for Raster Display Histogram Equalization and U.S. Pat. No. 3,996,421 to Pruznick et al for Television Display Utilizing Local Area Brightness Control. These references operate directly upon the video data without the use of auxiliary scanners or modification of the image sensor or TV camera.
Ketchem U.S. Pat. No. 3,983,320 teaches a digital system which performs video equlization on a local area or sliding window basis. The intensity of a given pixel or point in the image is adjusted according to a histogram of the area contained in a window surrounding this point. As the window is moved across the image field, the process is repeated for points within the window. Mini-histograms or sub-histograms are employed to provide statistical data, from which multiple truncation maps are formed. Such truncated video data provides enhanced gray level contrast over all portions of the picture image, regardless of the general brightness or darkness level of particular portions of the image. Such increased gray level contrast is provided by means of increased numbers of intensity levels within local areas of minimum contrast. In this way the overall image contrast for dark, gray or bright level areas is enhanced. However result is achieved at the expense of a substantially complex mechanization, including reliance upon a large number of multiplications.
Pruznick U.S. Pat. No. 3,996,421 uses a "sliding window" similarly as Ketchem U.S. Pat. No. 3,983,320, but does not employ the truncated histogram. Instead, Pruznick determines the average image intensity within the window at a given time, and enhances the intensity differences between given pixels or points therein and such average. Such enhancement or equalization is performed by both adjusting the gain of such differences and biasing the level of such intensity. In this way, an overly bright area is made less bright and the contrast therein improved within the dynamic limits of the display, a dark area is brightened and its contrasts enhanced, and gray areas subjected to contrast improvement with little or no bias adjustment in average brightness. However, Pruznick, like Ketchem, employs a complex mechanization relying upon a large number of multiplications.
Accordingly, a disadvantage of the prior art digital processing for image contrast enhancement has been the reliance upon complex modes of mechanization relying on large numbers of multiplications, as to increase equipment cost, reduce system reliability and adversely affect system processing speeds.